FIG. 11 is a vertical sectional view illustrating a brushless flat motor which is an example of an electrically powered apparatus according to prior art. A base 21 and a base plate 22 for attaching a stator coil are connected together by stator bolts 23 to provide a frame construction. A rotor 27 having a rotor magnet fixed thereto is secured to a motor shaft 26 rotatably supported by bearings 24 and 25 which are fixed to the base 21 and the base plate 22 respectively. A stator coil 28 is secured to the base plate 22 in relation to the rotor 27 with the base plate 22 therebetween and secured to the base plate 22 are a tachogenerator 33 to be activated by the rotor 27 and a Hall element 29 adapted to control the rotational speed of the rotor. A construction similar to the above may also be applied to a sheet coil motor having a motor coil manufactured by a photoetching method to allow the stator bolts to be made very thin.
The electrically powered apparatus of the construction explained above may be utilized in many ways, for example, as a laser scanner with a polygon mirror fixed to the motor shaft 26, an optical disc driving means with an optical disc removably attached directly to the rotor 27 or a blower with the blades of a fan fixedly mounted to the rotor 27.
In the electrically powered apparatus constructed as above, the clearance of the bearings 24 and 25 in the radial direction is so large that the rotor 27 may be vibrated, particularly where ball bearings are used for the bearings 24 and 25, and and vibration may thus be caused, such vibration causing problems in tape recorders and VTR which are susceptible to vibration.
In addition, it is to be noted that there are no radial bearings available which display good resistance to corrosion without the use of sealing means and they thus cannot be used in a submerged condition. Such bearings also exhibit poor resistance characteristics under circumstances where dew is a frequent occurrence.
It is further to be noted that when the motors are to be utilized in electric appliances, uneven rotation causes problems, and since the frictional resistance varies considerably in the case of radial bearings, uneven rotation may result.
For electrically powered apparatus of the sort described above, it is preferable in many cases that such apparatuses be short in axial length, as well as being thin. However, the motor cannot be made thin unless the distance between the radial bearings 24 and 25 can be made short. Conversely if the distance between the radial bearings is made short, the radial load to be supported by the bearings will be increased and the possibility of tilting of the motor shaft will also be increased due to the clearance existing in the bearings.
It is further to be noted that with the electrically powered apparatus of the type described above, the thrust load is always applied by the magnet of the rotor, and the metallic balls and the retainer of the ball bearings are subjected to permanent deformation if the apparatus is left unused for a long period of time, making it impossible for the apparatus to be started again. It is also to be noted that when the electrically powered apparatus is used for a high speed laser scanner which is adapted to reproduce an image of high resolution, the motor shaft 26 as well as the bearings 24 and 25 have to be machined extremely precisely, so as to minimize deflection of the plane of the polygon mirror, and at the same time they have to be assembled with the polygon mirror with high precision.